It is known in the art of steel making to continuously cast molten steel using an oscillating mold, typically a water cooled copper-faced mold, having a straight or curved channel. The mold typically has a rectangular horizontal cross-section as wide as the slab to be cast and with relatively narrow ends that are the thickness of the slab to be cast. Liquid steel in the upper portion of the mold is cooled as it moves downward through the water cooled mold, generating a steel shell as it passes through the mold before exiting the mold at the bottom. The molten steel enters the mold from a tundish through an entry nozzle submerged in the liquid steel in the mold. The submerged entry nozzle is normally located generally centrally of the mold cross-section, and is provided with opposed exit ports that direct liquid steel outwardly toward the narrow sides of the mold, upwardly, downwardly or vertically at 90 degrees to the submerged entry nozzle.
Without a magnetic brake, the flow of liquid steel out of the submerged entry nozzle varies in direction and velocity due to various external conditions. This can create disturbances in the steel flow that affect the slab surface quality and slab internal quality. These disturbances tend to generate undesired temperature imbalances that interfere with uniform solidification of the steel as it passes through the mold and downstream thereof, and also increase the tendency of the steel to incorporate unwanted inclusions from the mold powder/slag/impurities mixture at the meniscus of the liquid steel at the top of the mold. A conventional magnetic brake inhibits these disturbances by reducing the velocity of liquid steel emanating from the submerged entry nozzle, thereby tending to constrict the eddies and prevent them from reaching the end edges of the mold and the upper surface of the pool of liquid steel at the top of the mold.
A conventional magnetic brake is typically comprised of a magnetic circuit energized by direct or slowly varying electric current passing through windings around an iron core. The magnetic circuit passes through the wide faces of the mold so as to provide a magnetic field through the interior of the mold. Normally, in a conventional magnetic brake the magnetic circuit passes through the mold about mid-way along the longitudinal length of the mold and extends so as to overlap the point of entry of liquid steel into the mold from the submerged entry nozzle, but does not extend up to the top of the liquid steel pool nor down to the bottom of the mold.
Although the magnetic field in a conventional magnetic brake can be varied (by varying the amount of current flowing through the windings around the iron core of the magnetic circuit) there is, nevertheless, typically no fine control over the manner in which the magnetic field is applied. Such fine control would improve the ability to control the flow characteristics of the steel as it exits from the submerged entry nozzle in the interest of generating uniform solidification of the shell of cast steel emerging from the mold and in the interest of reducing unwanted inclusion and non-uniform surface effects.
Attempts have been made by various prior workers in the field to provide some variation in the magnetic field applied through the mold. Representative such attempts are disclosed, for example, in U.S. Pat. Nos. 5,404,933 and 5,613,548.